Bidirectional switching device and bidirectional switching circuit using the same

ABSTRACT

A bidirectional switching device includes a semiconductor multilayer structure made of a nitride semiconductor, a first ohmic electrode and a second ohmic electrode which are formed on the semiconductor multilayer structure, and a first gate electrode and a second gate electrode. The first gate electrode is covered with a first shield electrode having a potential substantially equal to that of the first ohmic electrode. The second gate electrode is covered with the second shield electrode having a potential substantially equal to that of the second ohmic electrode. An end of the first shield electrode is positioned between the first gate electrode and the second gate electrode, and an end of the second shield electrode is positioned between the second gate electrode and the first gate electrode.

CROSS-REFERENCE TO RELATED APPLICATION

This is a continuation of PCT International ApplicationPCT/JP2010/007252 filed on Dec. 14, 2010, which claims priority toJapanese Patent Application No. 2010-072520 filed on Mar. 26, 2010. Thedisclosures of these applications including the specifications, thedrawings, and the claims are hereby incorporated by reference in theirentirety.

BACKGROUND

The present disclosure relates to bidirectional switching devices andbidirectional switching circuits using such bidirectional switchingdevices.

There has been a demand for electronic equipment which saves moreenergy, and it has been desired to improve the power conversionefficiency of power converters, such as a power supply, an inverter, amatrix converter, etc., which consume a large amount of power. Inparticular, a matrix converter directly converting AC power into ACpower having a different frequency and voltage can convert AC powerwithout conduction through a diode rectifier, and therefore, it can beexpected to improve the power conversion efficiency, compared toconventional inverters. The matrix converter includes a bidirectionalswitch conducting a current flowing in two directions, and having abreakdown voltage with respect to positive and negative voltages. Abidirectional switch currently generally used includes two insulatedgate bipolar transistors (IGBTs) connected in antiparallel, and twodiodes each of which is connected to the corresponding one of the IGBTsin series.

It is important for a semiconductor element performing a bidirectionalswitching to reduce a switching loss expressed by a product of atransient voltage and a transient current at a time of switching, and aconduction loss consumed by a resistance of the semiconductor elementitself (referred to as an on-state resistance) in the on state. However,when a bidirectional switching circuit is formed by using a silicon (Si)device, it has been difficult to reduce the on-state resistance due to aSi material limit.

In order to reduce the conduction loss beyond the Si material limit, ithas been contemplated to introduce a semiconductor element using awide-gap semiconductor made of a nitride semiconductor (e.g., galliumnitride (GaN) etc.), silicon carbide (SiC), etc. The wide-gapsemiconductor has a dielectric strength higher than that of Si by aboutan order of magnitude. In particular, charge occurs at a heterojunctioninterface between aluminum gallium nitride (AlGaN) and gallium nitride(GaN) due to spontaneous polarization and piezoelectric polarization. Asa result, even if the layers are undoped, a two-dimensional electron gas(2DEG) layer is formed which has a sheet carrier concentration of 1×10¹³cm⁻² or more and a mobility of as high as 1000 cm² V/sec or more.Therefore, an AlGaN/GaN heterojunction electric field effect transistor(AlGaN/GaN-HFET) has been expected to serve as a power switchingtransistor which achieves a low on-state resistance and a high breakdownvoltage.

However, as well as conventional bidirectional switching circuits, evenif an AlGaN/GaN-HFET is used for a bidirectional switching circuit, itis necessary to provide two AlGaN/GaN-HFETs and two diodes, and comparedto the Si device, significant reduction of the on-state resistancecannot be expected.

In order to achieve a bidirectional switch having a lower on-stateresistance, for example, International Patent Publication No. WO08/062,800 proposes a bidirectional switching device which serves as asemiconductor element having double gates and in which one element canconstitute a bidirectional switch.

SUMMARY

However, the present inventors have found a problem where, if abidirectional switching device having double gates performs a switchingoperation, gate noise is generated, resulting in an unstable switchingoperation.

It is an object of the present disclosure to solve the problem found bythe present inventors where, if a bidirectional switching device havingdouble gates performs a switching operation, the switching operationbecomes unstable, and to achieve a bidirectional switching device stablyperforming the operation.

In order to attain the above object, the present disclosure is directedto a bidirectional switching device including a first shield electrodeand a second shield electrode shielding lines of electric forcegenerated between a first gate electrode and a second gate electrode.

Specifically, the bidirectional switching device of the presentdisclosure includes: a semiconductor multilayer structure formed on asubstrate and made of a nitride semiconductor; a first ohmic electrodeand a second ohmic electrode formed on the semiconductor multilayerstructure to be spaced from each other with an interval therebetween; afirst gate electrode formed between the first ohmic electrode and thesecond ohmic electrode; a second gate electrode formed between the firstgate electrode and the second ohmic electrode; a first insulating layerformed on the semiconductor multilayer structure to cover the first gateelectrode and the second gate electrode; a first shield electrode formedon the first insulating layer to cover the first gate electrode, andhaving a potential equal to that of the first ohmic electrode; and asecond shield electrode formed on the first insulating layer to coverthe second gate electrode, and having a potential equal to that of thesecond ohmic electrode, wherein an end of the first shield electrode ispositioned between the first gate electrode and the second gateelectrode, and an end of the second shield electrode is positionedbetween the second gate electrode and the first gate electrode.

The bidirectional switching device of the present disclosure can shieldmost part of lines of electric force generated between the first gateelectrode and the second gate electrode. Therefore, a parasiticcapacitance between the first gate electrode and the second gateelectrode can be reduced. As a result, gate noise generated at a time ofswitching can be reduced, thereby making it possible to achieve abidirectional switching device which stably performs an operation.

In the bidirectional switching device of the present disclosure, aminimum distance between the semiconductor multilayer structure and partof the first shield electrode positioned between the first gateelectrode and the second gate electrode may be smaller than a distancebetween an upper surface of the semiconductor multilayer structure andan upper surface of the first gate electrode, and a minimum distancebetween the semiconductor multilayer structure and part of the secondshield electrode positioned between the second gate electrode and thefirst gate electrode may be smaller than a distance between the uppersurface of the semiconductor multilayer structure and an upper surfaceof the second gate electrode. Such a structure can shield the lines ofelectric force generated between the first gate electrode and the secondgate electrode.

In this case, the minimum distance between the semiconductor multilayerstructure and the part of the first shield electrode positioned betweenthe first gate electrode and the second gate electrode may be smallerthan a minimum distance between the first gate electrode and the firstshield electrode, and the minimum distance between the semiconductormultilayer structure and the part of the second shield electrodepositioned between the second gate electrode and the first gateelectrode may be smaller than a minimum distance between the second gateelectrode and the second shield electrode. Such a structure can improvean advantage of reducing the parasitic capacitance while maintaining abreakdown voltage between the ohmic electrode and the gate electrode.

The bidirectional switching device of the present disclosure may furtherincludes a second insulating layer formed on the first insulating layerand having a thickness larger than that of the first insulating layer,wherein part of the first insulating layer located between the firstgate electrode and the second gate electrode has an upper surfacelocated below the upper surface of the first gate electrode and theupper surface of the second gate electrode, the first shield electrodeincludes: a first metal layer formed on the first insulating layer to bepositioned between the first gate electrode and the second gateelectrode, and covered with the second insulating layer; and a secondmetal layer formed on the second insulating layer, and connected to thefirst metal layer in an opening formed in the second insulating layer,and the second shield electrode includes: a third metal layer formed onthe first insulating layer to be positioned between the second gateelectrode and the first gate electrode, and covered with the secondinsulating layer; and a fourth metal layer formed on the secondinsulating layer, and connected to the third metal layer in an openingformed in the second insulating layer.

The bidirectional switching device of the present disclosure may furtherinclude: a first p-type nitride semiconductor layer formed between thefirst gate electrode and the semiconductor multilayer structure; and asecond p-type nitride semiconductor layer formed between the second gateelectrode and the semiconductor multilayer structure. In this case, theminimum distance between the semiconductor multilayer structure and thepart of the first shield electrode positioned between the first gateelectrode and the second gate electrode may be smaller than a distancebetween the upper surface of the semiconductor multilayer structure andan upper surface of the first p-type nitride semiconductor layer, andthe minimum distance between the semiconductor multilayer structure andthe part of the second shield electrode positioned between the secondgate electrode and the first gate electrode may be smaller than adistance between the upper surface of the semiconductor multilayerstructure and an upper surface of the second p-type nitridesemiconductor layer.

A bidirectional switching circuit of the present disclosure includes:the bidirectional switching device of the present disclosure; a firstgate driving circuit connected to the first gate electrode through afirst gate resistance; and a second gate driving circuit connected tothe second gate electrode through a second gate resistance.

According to the bidirectional switching device of the presentdisclosure, a bidirectional switching device in which gate noise isreduced, and which stably perform an operation can be achieved.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a circuit diagram explaining problems which can occur in abidirectional switching device.

FIG. 2 is a view showing waveforms of gate voltages in the bidirectionalswitching circuit of FIG. 1.

FIG. 3 is a cross-sectional view showing a bidirectional switchingdevice according to one embodiment.

FIG. 4 is a graph showing a parasitic capacitance of the bidirectionalswitching device according to the one embodiment and a parasiticcapacitance of a bidirectional switching device.

FIG. 5 is a cross-sectional view of a modified example of thebidirectional switching device according to the one embodiment.

FIG. 6 is a circuit diagram showing a bidirectional switching circuitaccording to the one embodiment.

DETAILED DESCRIPTION

First, a problem occurring in a semiconductor element having doublegates which was found by the present inventors will be described. FIG. 1shows a circuit configuration when driving a GaN based bidirectionalswitching device 101. In FIG. 1, a first gate G1 of the bidirectionalswitching device 101 is connected to a first gate driving circuit 102Athrough a gate resistance Rg, and a second gate G2 is connected to asecond gate driving circuit 102B through a gate resistance Rg. The firstgate driving circuit 102A applies a bias voltage to the first gate G1 bya first power supply 103A, and the second gate driving circuit 102Bapplies a bias voltage to the second gate G2 by a second power supply103B. FIG. 1 shows a diode mode in which the first gate G1 is in the offstate, the second gate G2 is in the on state, and the bidirectionalswitching device 101 itself serves as a diode.

Switching will be described below which causes a transition from a statewhere a current flowing from a first source S1 to a second source S2 isconducted to a state where a current flowing from the second source S2to the second source S1 is blocked in the diode mode will be considered.With the transition of the state, a potential V_(s2s1) between thesecond source S2 and the first source S1 changes from, e.g., −2 V to 100V. Along this change, a charge/discharge current flows from a parasiticcapacitance C_(gg) between the first gate G1 and the second gate G2. Thecharge/discharge current flows, as shown in FIG. 1, through the gateresistance Rg connected between the first gate driving circuit 102A andthe first gate G1, and through the gate resistance Rg connected betweenthe second gate driving circuit 102B and the second gate G2. Therefore,a voltage is momentarily generated at the gate resistance Rg.

Originally, a first gate voltage V_(g1) applied to the first gate G1 hasto be maintained at, e.g., 0V which is on the off state, and a secondgate voltage V_(g2) applied to the second gate G2 has to be maintainedat, e.g., 4 V which is on the on state. However, if a voltage V_(s2s1)changes between the second source S2 and the first source S1, gate noiseis generated in the first gate voltage V_(g1) and the second gatevoltage V_(g2) by the charge/discharge current of the parasiticcapacitance C_(gg) and the gate resistance Rg, as shown in FIG. 2. Ifnegative voltage noise is generated when the gate voltage is positive,such negative voltage noise is opposite to the voltage applied betweenthe gate and the source of the bidirectional switching device 101, andthus, increases the possibility of causing breakdown of thebidirectional switching device 101. If positive voltage noise isgenerated when the gate voltage is 0 V and the gate voltage exceeds athreshold voltage, this leads to false firing of the bidirectionalswitching device 101. This short-circuits a power supply, increasing thepossibility of destroying the device. In this way, due to the gate noisegenerated by the parasitic capacitance C_(gg), it becomes difficult toallow the bidirectional switching device to stably perform the switchingoperation. As a capacitance value of the parasitic capacitance C_(gg)becomes larger, the gate noise becomes larger, and the false firingbecomes more likely to occur. Therefore, in order to stably perform aswitching operation, it is important to reduce the capacitance value ofthe parasitic capacitance C_(gg) as much as possible.

In view of the above finding, the present inventors have developed abidirectional switching device which can reduce the capacitance value ofthe parasitic capacitance C_(gg), and stably perform a switchingoperation. The bidirectional switching device which reduces thecapacitance value of the parasitic capacitance C_(gg) will be describedin detail below by using an embodiment.

(One Embodiment)

FIG. 3 shows a cross-sectional structure of a bidirectional switchingdevice according to one embodiment. As shown in FIG. 3, a semiconductormultilayer structure 203 is formed on a conductive substrate 201 made ofsilicon (Si) with a buffer layer 202 made of aluminum nitride (AlN) andhaving a thickness of 100 nm being interposed therebetween. Thesemiconductor multilayer structure 203 includes a first layer 205 madeof an undoped gallium nitride (GaN) having a thickness of about 2 μm,and a second layer 206 made of an undoped aluminum gallium nitride(AlGaN) having a thickness of about 20 nm, the first layer 205 and thesecond layer 206 being sequentially laminated in this order from thebottom.

Charge occurs in the vicinity of a heterointerface between the firstlayer 205 and the second layer 206 due to spontaneous polarization andpiezoelectric polarization. As a result, a channel region is formedwhich is a two-dimensional electron gas (2DEG) layer having a sheetcarrier concentration of 1×10¹³ cm⁻² or more and a mobility of 1000 cm²V/sec or more.

A first ohmic electrode 211 and a second ohmic electrode 212 are formedon the semiconductor multilayer structure 203 to be spaced from eachother with an interval therebetween. Each of the first ohmic electrode211 and the second ohmic electrode 212 includes a laminated layer oftitanium (Ti) and aluminum (Al), and forms an ohmic contact with achannel region. In FIG. 3, in order to reduce the contact resistance,for example, part of the second layer 206 is removed, and the firstlayer 205 is hollowed out to a depth of about 40 nm, whereby the firstohmic electrode 211 and second ohmic electrode 212 contact an interfacebetween the second layer 206 and the first layer 205.

In a region between the first ohmic electrode 211 and the second ohmicelectrode 212 on the semiconductor multilayer structure 203, a firstgate electrode 217 is formed on the semiconductor multilayer structure203 with a first p-type nitride semiconductor layer 215 interposedtherebetween, and a second gate electrode 218 is formed on thesemiconductor multilayer structure 203 with a second p-type nitridesemiconductor layer 216 interposed therebetween, the first gateelectrode 217 and the second gate electrode 218 being sequentiallyformed in this order from a side closer to the first ohmic electrode211. The second gate electrode 218 is formed between the first gateelectrode 217 and the second gate electrode 212. A distance between thefirst p-type nitride semiconductor layer 215 and the second p-typenitride semiconductor layer 216 is designed such that the semiconductordevice can withstand the maximum voltage to be applied to thesemiconductor device. The first gate electrode 217 includes a laminatedlayer of palladium (Pd) and gold (Au), and forms an ohmic contact withthe first p-type nitride semiconductor layer 215. Similarly, the secondgate electrode 218 includes a laminated layer of palladium (Pd) and gold(Au), and forms an ohmic contact with the second p-type nitridesemiconductor layer 216.

The first p-type nitride semiconductor layer 215 and the second p-typenitride semiconductor layers 216 have a thickness of 300 nm, and aremade of p-type GaN doped with magnesium (Mg). The first p-type nitridesemiconductor layer 215 and the second layer 206 form a pn junction, andthe second p-type nitride semiconductor layer 216 and the second layer206 form a pn junction. As a result, even when a voltage applied to thefirst gate electrode 217 and the second gate electrode 218 is 0 V, thesecond layer 206 and the first layer 205 include a depletion layertherein from the boundary with the first p-type nitride semiconductorlayer 215 or the second p-type nitride semiconductor layer 216 towardthe substrate 201, and the first ohmic electrode 211 or the second ohmicelectrode 212. Therefore, even when a voltage applied to the first gateelectrode 217 and the second gate electrode 218 is 0 V, a currentflowing through the channel region is blocked, so that a normally-offoperation can be performed. In the case of the bidirectional switchingdevice of the embodiment, threshold voltages of the first gate electrode217 and the second gate electrode 218 are approximately 1 V.

If a gate voltage of 3 V or more which exceeds a built-in potential ofthe pn junction is applied to the first gate electrode 217 and thesecond gate electrode 218, holes can be implanted into the channelregion. Since the mobility of holes in a nitride semiconductor is farlower than that of electrons, holes implanted into the channel regionhardly contribute as a carrier for allowing a current to flow.Therefore, the implanted holes serve as donor ions which improve anadvantage of generating the same number of electrons as the holes in thechannel region, and generating the electrons inside the channel region.In other words, it becomes possible to modulate the carrierconcentration in the channel region, thereby making it possible toachieve a normally off type bidirectional switching device providing alarger operating current and a lower resistance.

The parasitic capacitance C_(gg) in FIG. 1 is a capacitance generatedbetween the first gate electrode 217 and the second gate electrode 218,and is determined by the number of lines of electric force generatedbetween the first gate electrode 217 and the second gate electrode 218.Therefore, the lines of electric force are shielded between the firstgate electrode 217 and the second gate electrode 218, thereby making itpossible to reduce the capacitance value of the parasitic capacitanceC_(gg).

The bidirectional switching device in the embodiment includes a firstshield electrode 221 and a second shield electrode 222 each made of gold(Au), etc., to reduce the number of the lines of electric force betweenthe first gate electrode 217 and the second gate electrode 218. Thefirst shield electrode 221 is connected to the first ohmic electrode211, and has a potential substantially equal to that of the first ohmicelectrode 211. The first shield electrode 221 is formed to cover thefirst gate electrode 217 through a first insulating layer 208, and havean end positioned closer to the second gate electrode 218 than the firstgate electrode 217 is. The second shield electrode 222 is connected tothe second ohmic electrode 212, and has a potential substantially equalto that of the second ohmic electrode 212. The second shield electrode222 is formed to cover the second gate electrode 218 through the firstinsulating layer 208, and have an end positioned closer to the firstgate electrode 217 than the second gate electrode 218 is. The end of thefirst shield electrode 221 is positioned between the first gateelectrode 217 and the second gate electrode 218. The end of the secondshield electrode 222 is positioned between the second gate electrode 218and the first gate electrode 217.

The first insulating layer 208 is made of silicon nitride (SiN), etc.,and is formed on the semiconductor multilayer structure 203 to coverpart of the first ohmic electrode 211, part of the second ohmicelectrode 212, the first gate electrode 217, and the second gateelectrode 218. The first insulating layer 208 has an opening forexposing the first ohmic electrode 211, and an opening for exposing thesecond ohmic electrode 212. The first shield electrode 221 is connectedto the first ohmic electrode 211 in the opening, and the second shieldelectrode 222 is connected to the second ohmic electrode 212 in theopening. The first shield electrode 221 and the second shield electrode222 are insulated from each other, and a second insulating layer 209made of SiN, etc., is formed to cover the first shield electrode 221 andthe second shield electrode 222.

The first shield electrode 221 also serves as a first ohmic electrodeline connecting the first ohmic electrode 211 and a first ohmicelectrode pad (not shown) together. The second shield electrode 222 alsoserves as a second ohmic electrode line connecting the second ohmicelectrode 212 and the first a second ohmic electrode pad (not shown)together. The first ohmic electrode pad corresponds to the first sourceS1 in FIG. 1, and the second ohmic electrode pad corresponds to thesecond source S2. The first gate electrode 217 is connected to the firstgate electrode pad (not shown) corresponding to the first gate G1 inFIG. 1, and the second gate electrode 218 is connected to the secondgate electrode pad (not shown) corresponding to the second gate G2.

The first shield electrode 221 and the second shield electrode 222 canshield at least part of the lines of electric force generated betweenthe first gate electrode 217 and the second gate electrode 218.Therefore, the capacitance value of the parasitic capacitance C_(gg) canbe reduced. In order to shield the lines of electric force, the firstshield electrode 221 has to cover the first gate electrode 217, and thesecond shield electrode 222 has to cover the second gate electrode 218.In order to efficiently shield the lines of electric force, it ispreferable that the end of the first shield electrode 221 be positionedcloser to the second gate electrode 218 than an end of the first gateelectrode 217 is, the end of the first gate electrode 217 beingpositioned closer to the second gate electrode 218. The end of thesecond shield electrode 222 be positioned closer to the first gateelectrode 217 than an end of the second gate electrode 218 is, the endof the second gate electrode 218 being positioned closer to the firstgate electrode 217.

It is preferable that the minimum distance between the semiconductormultilayer structure 203 and part of the first shield electrode 221positioned between the first gate electrode 217 and the second gateelectrode 218 be smaller than a distance from the upper surface of thesemiconductor multilayer structure 203 to the upper surface of the firstgate electrode 217. Similarly, it is preferable that the minimumdistance between the semiconductor multilayer structure 203 and part ofthe second shield electrode 222 positioned between the second gateelectrode 218 and the first gate electrode 217 be smaller than adistance from the upper surface of the semiconductor multilayerstructure 203 to the upper surface of the second gate electrode 218.Specifically, it is preferable that the lower surface of the end of thefirst shield electrode 221 is positioned closer to the semiconductormultilayer structure 203 than the upper surface of the first gateelectrode 217 is (positioned below the upper surface of the first gateelectrode 217), and the lower surface of the end of the second shieldelectrode 222 is positioned closer to the semiconductor multilayerstructure 203 than the upper surface of the second gate electrode 218 is(positioned below the upper surface of the second gate electrode 218).

In the embodiment, the first gate electrode 217 and the second gateelectrode 218 are formed on the first p-type nitride semiconductor layer215 and the second p-type nitride semiconductor layer 216, respectively.Therefore, the minimum distance between the first shield electrode 221and the semiconductor multilayer structure 203 in the end of the firstshield electrode 221 is smaller than a distance from the upper surfaceof the semiconductor multilayer structure 203 to the upper surface ofthe first p-type nitride semiconductor layer 215. Similarly, the minimumdistance between the second shield electrode 222 and the semiconductormultilayer structure 203 in the end of the second shield electrode 222is smaller than a distance from the upper surface of the semiconductormultilayer structure 203 to the upper surface of the second p-typenitride semiconductor layer 216. Therefore, the end of the first shieldelectrode 221 is positioned closer to the semiconductor multilayerstructure 203 than the lower surface of the first gate electrode 217 is,and the end of the second shield electrode 222 is positioned closer tothe semiconductor multilayer structure 203 than the lower surface of thesecond gate electrode 218 is. This positional relationship canefficiently shield the lines of electric force generated between thefirst gate electrode 217 and the second gate electrode 218.

FIG. 4 shows a comparison of the parasitic capacitance C_(gg) of thebidirectional switching device of the embodiment and a parasiticcapacitance C_(gg) of a bidirectional switching device having no shieldelectrodes. In FIG. 4, a horizontal axis represents a voltage V_(s2s1)between the second source S2 and the first source 51, and a verticalaxis represents the capacitance values of the parasitic capacitancesC_(gg). As shown in FIG. 4, when the voltage V_(s2s1) is higher, each ofthe capacitance values of the parasitic capacitances C_(gg) becomessmaller. The capacitance value of the parasitic capacitance C_(gg) ofthe bidirectional switching device having the shield electrodes issmaller than that of the bidirectional switching device having no shieldelectrodes. The existence of the shield electrodes can achieve thebidirectional switching device in which gate noise is reduced, and afalse firing is less likely to occur, and which can perform a switchingoperation more stably.

Providing the first shield electrodes 221 and the second shieldelectrodes 222 makes it possible not only to reduce the capacitancevalue of the parasitic capacitance C_(gg), but also to increase acapacitance value of a parasitic capacitance C_(gs1) between the firstohmic electrode 211 and the first gate electrode 217, and a capacitancevalue of a parasitic capacitance C_(gs2) between the second ohmicelectrode 212 and the second gate electrode 218. Increase of thecapacitance value of the parasitic capacitance C_(gs1) and thecapacitance value of the parasitic capacitance C_(gs2) makes it possibleto reduce an impedance between the first gate G1 and the first sourceS1, and an impedance between the second gate G2 and the second sourceS2. Therefore, the gate noise which is a high frequency component can bereduced.

In the embodiment, the first shield electrode 211 and the second shieldelectrode 222 are formed on the first insulating layer 208. Therefore,the minimum distance between the first gate electrode 217 and the firstshield electrode 221, the minimum distance between the second gateelectrode 218 and the second shield electrode 222, the minimum distancebetween the first shield electrode 221 and the semiconductor multilayerstructure 203, and the minimum distance between the second shieldelectrode 222 and the semiconductor multilayer structure 203 aredetermined by the thickness of the first insulating layer 208, and havesubstantially the same value. In order to efficiently shield the linesof electric force, it is preferable that the distance between the lowersurface of the end of the first shield electrode 221 and the uppersurface of the semiconductor multilayer structure 203 in the end of thefirst shield electrode 221, and the distance between the lower surfaceof the end of the second shield electrode 222 and the upper surface ofthe semiconductor multilayer structure 203 in the end of the secondshield electrode 222 be as small as possible. A breakdown voltagebetween the first gate electrode 217 and the first ohmic electrode 211,and a breakdown voltage between the second gate electrode 218 and thesecond ohmic electrode 212 are respectively determined by the distancebetween the first gate electrode 217 and the first shield electrode 221,and the distance between the second gate electrode 218 and the secondshield electrode 222. Therefore, it is preferable that the distancebetween the first gate electrode 217 and the first shield electrode 221,and the distance between the second gate electrode 218 and the secondshield electrode 222 be as large as possible. Therefore, the embodimentmay have a structure shown in FIG. 5.

As shown in FIG. 5, the first shield electrode 221 includes a firstmetal layer 221A and a second metal layer 221B, and the second shieldelectrode 222 includes a third metal layer 222A and a fourth metal layer222B. A first insulating layer 251 made of SiN and having a thickness ofapproximately 100 nm is formed on the second layer 206. In a regionbetween the first gate electrode 217 and the second gate electrode 218,the first metal layer 221A and the third metal layer 222A are formed onthe first insulating layer 251 to be spaced from each other ith aninterval therebetween. A second insulating layer 252 made of SiN andhaving a thickness of approximately 100 nm to 300 nm is formed on thesecond layer 206 to cover the first metal layer 221A and the third metallayer 222A. The second metal layer 221B connected to the first ohmicelectrode 211 and the first metal layer 221A, and the fourth metal layer222B connected to the second ohmic electrode 212 and the third metallayer 222A are formed on the second insulating layer 252. The firstmetal layer 221A and the second metal layer 221B form the first shieldelectrode 221, and the third metal layer 222A and the fourth metal layer222B form the second shield electrode 222. A third insulating layer 253made of SiN is formed to cover the first shield electrode 221 and thesecond shield electrode 222.

In the bidirectional switching device shown in FIG. 5, a distance d1between the lower surface of the end of the first shield electrode 221and the upper surface of semiconductor multilayer structure 203 in theend of the first shield electrode 221, and a distance d2 between thelower surface of the end of the second shield electrode 222 and theupper surface of semiconductor multilayer structure 203 in the end ofthe second shield electrode 222 are determined by the thickness of thefirst insulating layer 251. In contrast, each of a distance d3 betweenthe upper surface of the first gate electrode 217 and the lower surfaceof the first shield electrode 221, and a distance d4 between the uppersurface of the second gate electrode 218 and the lower surface of thesecond shield electrode 222 is the sum of the thickness of the firstinsulating layer 251 and the thickness of the second insulating layer252. Therefore, it is easy to decrease the distance d1 and the distanced2 while increasing the distance d3 and the distance d4.

It is preferable that the thickness of the first insulating layer 251 beas thin as possible only if the first shield electrode 221, the secondshield electrode 222, and the semiconductor multilayer structure 203 canbe insulated from one another. At least the thickness may beapproximately 10 nm, and in view of easy formation of the layer, thethickness may be approximately 50 nm to 100 nm. If the thickness of thesecond insulating layer 252 is larger, the breakdown voltage between thefirst gate electrode 217 and the first ohmic electrode 211, and thebreakdown voltage between the second gate electrode 218 and the secondohmic electrode 212 can be higher. The breakdown voltage between thefirst gate electrode 217 and the first ohmic electrode 211 is alsoinfluenced by a distance between the first ohmic electrode 211 and thefirst gate electrode 217 (or the first p-type nitride semiconductorlayer 215). Therefore, the distance between the first ohmic electrode211 and the first gate electrode 217, and the distance between the firstgate electrode 217 and the first shield electrode 221 may be equal toeach other. In general, the distance between the first ohmic electrode211 and the first gate electrode 217 is approximately 1 μm. In thiscase, the distance between the first gate electrode 217 and the firstshield electrode 221 is preferably approximately 1 μm, too. However, thedistance between the first ohmic electrode 211 and the first gateelectrode 217, and the distance between the first gate electrode 217 andthe first shield electrode 221 do not have to be equal to each other. Adistance between the second gate electrode 218 and the second ohmicelectrode 212 and a distance between the second gate electrode 218 andthe second shield electrode 222 may be formed in a manner similar to thedistance between the first gate electrode 217 and the first ohmicelectrode 211, and the distance between the first gate electrode 217 andthe first shield electrode 221.

FIG. 6 shows an example of a bidirectional switching circuit using thebidirectional switching device. The bidirectional switching circuitincludes a bidirectional switching device 301 according to theembodiment, a first gate driving circuit 302A for driving a first gateG1, and a second gate driving circuit 302B for driving a second gate G2.The first gate driving circuit 302A is connected to the first gate G1through a gate resistance Rg, and the second gate driving circuit 302Bis connected to the second gate G2 through a gate resistance Rg. A firstpower supply 303A is connected to the first gate driving circuit 302A,and a second power supply 303B is connected to the second gate drivingcircuit 302B. The first gate driving circuit 302A and the second gatedriving circuit 302B apply a bias voltage to the first gate G1 and thesecond gate G2, respectively, based on a driving signal.

The bidirectional switching circuit is formed by using the bidirectionalswitching device of the embodiment, thereby making it possible toachieve a bidirectional switching circuit in which gate noise is lesslikely to be generated, and which stably performs a switching operation.A resistance value of the gate resistance Rg has to be determined by aturn-on time and turn-off time of the gate resistance Rg. If theresistance value of the gate resistance Rg is larger, gate noise becomeslarger due to a charge/discharge current of the parasitic capacitanceC_(gg). Therefore, when using a conventional bidirectional switchingdevice having no shield electrodes, the value of the gate resistance Rgis limited. However, in the bidirectional switching device of theembodiment, the capacitance value of the parasitic capacitance C_(gg) isreduced, whereby the charge/discharge current can be reduced to a smallvalue. Therefore, the bidirectional switching device of the embodimentcan obtain an advantage that the resistance value of the gate resistanceRg can be set to have an optimum value.

The gate resistance Rg may be an internal resistance of the gate drivingcircuit 302. The first gate G1 and the second gate G2 are driven by thegate driving circuit 302, thereby making it possible to switch among abidirectional conduction operation mode in which a bidirectional currentflows between the first source S1 and second source S2, a bidirectionalconduction operation mode in which the bidirectional current is blocked,a first diode operation mode in which a current flows from the firstsource S1 to the second source S2, and a current flowing from the secondsource S2 to the first source S1 is blocked, and a second diodeoperation mode in which a current flows from the second source S2 to thefirst source S1, and a current flowing from the first source S1 to thesecond source S2 is blocked. Therefore, a power supply 305 and a load306 are connected between the first source S1 and the second source S2,thereby making it possible to easily control the operation of the load306. The combination of the bidirectional switching circuits form a halfbridge circuit, and the circuit can be applied to a power conversioncircuit, a motor control circuit and a driving circuit of a plasmadisplay, etc.

The embodiment shows the example in which the first gate electrode andthe second gate electrode are respectively formed on a first p-typenitride semiconductor layer and a second p-type nitride semiconductorlayer. The embodiment is not limited to such a structure, but may have astructure in which the first gate electrode and the second gateelectrode are joined to the second layer to form a Schottky junction, ora structure in which a gate insulating film is formed among the firstgate electrode, the second gate electrode, and the second layer. Havingthe structure in which the gate electrode is formed on the p-typenitride semiconductor layer obtains the following advantage. The p-typenitride semiconductor layer is generally set to have a thickness ofapproximately 100 nm to 300 nm. Therefore, if the thickness of the firstinsulating layer is approximately 50 nm, an end of the shield electrodecan be formed to be closer to the semiconductor multilayer structurethan the lower surface of the gate electrode is. Therefore, such astructure can improve the advantage of shielding lines of electric forcebetween the first gate electrode and the second gate electrode.

The embodiment shows the example of using the conductive Si substrate asthe substrate. If the substrate is conductive, the back surface of thesubstrate may be provided with a back electrode for stabilizing thepotential of the substrate. The back electrode may be a laminated filmof, e.g., made of nickel (Ni), chromium (Cr), and silver (Ag), andhaving a thickness of approximately 800 nm. The back electrode may beconnected to the first ohmic electrode or the second ohmic electrode,and may be fixed so as to have the same potential as the potential ofthe connected ohmic electrode. A circuit in which the potential of theback electrode is lower than a higher potential of a potential of thefirst ohmic electrode or a potential of the second ohmic electrode maybe provided. Such a circuit, unlike the case where the potential of thesubstrate is fixed to have the same potential as the potential of thefirst ohmic electrode or the second ohmic electrode, can prevent anunstable operation due to increase in asymmetry of the potentials of thesemiconductor element. Other than the Si substrate, a conductivesubstrate made of silicon carbide (SiC) or gallium nitride (GaN), etc.,may be used. A insulative substrate made of sapphire, etc., can be used.

In the embodiment, the first insulating layer, the second insulatinglayer, and the third insulating layer are made of SiN, but they may bemade of other insulating materials, such as aluminum nitride (AlN) orsilicon oxide (SiO₂), etc.

The bidirectional switching device and the bidirectional switchingcircuit of the present disclosure can achieve a bidirectional switchingdevice in which gate noise is reduced, and which stably performs anoperation, and in particular, are useful as a bidirectional switchingdevice used for a power conversion circuit, etc., and a bidirectionalswitching circuit using such a bidirectional switching device.

What is claimed is:
 1. A bidirectional switching device, comprising: asemiconductor multilayer structure formed on a substrate and made of anitride semiconductor; a first ohmic electrode and a second ohmicelectrode formed on the semiconductor multilayer structure to be spacedfrom each other with an interval therebetween; a first gate electrodeformed between the first ohmic electrode and the second ohmic electrode;a second gate electrode formed between the first gate electrode and thesecond ohmic electrode; a first insulating layer formed on thesemiconductor multilayer structure to cover the first gate electrode andthe second gate electrode; a first shield electrode formed on the firstinsulating layer to cover the first gate electrode, and having apotential equal to that of the first ohmic electrode; and a secondshield electrode formed on the first insulating layer to cover thesecond gate electrode, and having a potential equal to that of thesecond ohmic electrode, wherein an end of the first shield electrode ispositioned between the first gate electrode and the second gateelectrode, and an end of the second shield electrode is positionedbetween the second gate electrode and the first gate electrode.
 2. Thebidirectional switching device of claim 1, wherein a minimum distancebetween the semiconductor multilayer structure and part of the firstshield electrode positioned between the first gate electrode and thesecond gate electrode is smaller than a distance between an uppersurface of the semiconductor multilayer structure and an upper surfaceof the first gate electrode, and a minimum distance between thesemiconductor multilayer structure and part of the second shieldelectrode positioned between the second gate electrode and the firstgate electrode is smaller than a distance between the upper surface ofthe semiconductor multilayer structure and an upper surface of thesecond gate electrode.
 3. The bidirectional switching device of claim 2,wherein the minimum distance between the semiconductor multilayerstructure and the part of the first shield electrode positioned betweenthe first gate electrode and the second gate electrode is smaller than aminimum distance between the first gate electrode and the first shieldelectrode, and the minimum distance between the semiconductor multilayerstructure and the part of the second shield electrode positioned betweenthe second gate electrode and the first gate electrode is smaller than aminimum distance between the second gate electrode and the second shieldelectrode.
 4. The bidirectional switching device of claim 3, furthercomprising a second insulating layer formed on the first insulatinglayer and having a thickness larger than that of the first insulatinglayer, wherein part of the first insulating layer located between thefirst gate electrode and the second gate electrode has an upper surfacelocated below the upper surface of the first gate electrode and theupper surface of the second gate electrode, the first shield electrodeincludes: a first metal layer formed on the first insulating layer to bepositioned between the first gate electrode and the second gateelectrode, and covered with the second insulating layer; and a secondmetal layer formed on the second insulating layer, and connected to thefirst metal layer in an opening formed in the second insulating layer,and the second shield electrode includes: a third metal layer formed onthe first insulating layer to be positioned between the second gateelectrode and the first gate electrode, and covered with the secondinsulating layer; and a fourth metal layer formed on the secondinsulating layer, and connected to the third metal layer in an openingformed in the second insulating layer.
 5. The bidirectional switchingdevice of claim 2, further comprising: a first p-type nitridesemiconductor layer formed between the first gate electrode and thesemiconductor multilayer structure; and a second p-type nitridesemiconductor layer formed between the second gate electrode and thesemiconductor multilayer structure.
 6. The bidirectional switchingdevice of claim 5, wherein the minimum distance between thesemiconductor multilayer structure and the part of the first shieldelectrode positioned between the first gate electrode and the secondgate electrode is smaller than a distance between the upper surface ofthe semiconductor multilayer structure and an upper surface of the firstp-type nitride semiconductor layer, and the minimum distance between thesemiconductor multilayer structure and the part of the second shieldelectrode positioned between the second gate electrode and the firstgate electrode is smaller than a distance between the upper surface ofthe semiconductor multilayer structure and an upper surface of thesecond p-type nitride semiconductor layer.
 7. A bidirectional switchingcircuit, comprising: the bidirectional switching device of claim 1; afirst gate driving circuit connected to the first gate electrode througha first gate resistance; and a second gate driving circuit connected tothe second gate electrode through a second gate resistance.
 8. Thebidirectional switching device of claim 1, wherein the semiconductormultilayer structure includes a first nitride semiconductor layer, and asecond nitride semiconductor layer formed on the first nitridesemiconductor layer, the second nitride semiconductor layer having alarger band gap energy than the first nitride semiconductor layer. 9.The bidirectional switching device of claim 8, wherein the first nitridesemiconductor layer is made of an undoped GaN, and the second nitridesemiconductor layer is made of Al_(x)Ga_(l-x) N (where 0<x<1).
 10. Thebidirectional switching device of claim 1, wherein a thickness of thefirst insulating layer is 50 nm to 100 nm.
 11. The bidirectionalswitching device of claim 4, wherein a thickness of the secondinsulating layer is 100 nm to 300 nm.
 12. The bidirectional switchingdevice of claim 5, wherein the first p-type nitride semiconductor layerand the second p-type nitride semiconductor layer are thicker than thefirst insulating layer respectively.
 13. The bidirectional switchingdevice of claim 5, wherein thicknesses of the first p-type nitridesemiconductor layer and the second p-type nitride semiconductor layerare 100 nm to 300 nm respectively.